Yes, we charge lipos to peak at 8.4volts.
But as soon as you connect a load (e.g. motor), some batteries will supply an average of only 7.2volts, some 7.8v, etc.
From my experience, the newer batteries supply higher voltage under load. Which might explain why people prefer 50c (i.e. newer battery technology) than old 25c.
You can do a simple test. Connect your race motor to a slave motor. Put a voltmeter at the output of the slave motor.
Then put a 25c battery to your race motor, take the voltage reading at the output of the slave motor.
Then do the same test with a 50c. Chances are you will get higher voltage reading at the output of the slave motor with a 50c, meaning the slave motor is spinning faster with a 50c.